Many studies, in bacteria as well as in other microbial organisms, have shown that some of the repair-deficient strains are more mutable than the wild-type. In studies with the ad-3 test system of Neurospora crassa, Dr. de Serres and his coworkers showed that the repair-deficient strains, upr-1 and uvs-2, are more mutable than wild type after UV or gamma-ray irradiation. It also has been shown by de Serres that the spectra of genetic alterations caused by UV in repair-deficient strains are different from that caused by UV in the wild type. It is of interest, therefore, to study and compare the mutagenic sensitivity of repair-deficient and wild-type strains to various chemical carcinogens and to compare the spectra of genetic alterations caused by the carcinogenic agents in the UV-sensitive strains and in the wild-type strain. The results of mutation induction experiments with MNNG indicate that both uvs-3 and uvs-5 strains are slightly less mutable than wild-type whereas uvs-6 shows a sensitivity similar to wild type. By contrast, however, upr-1 is two times and uvs-2 is five times more sensitive than wild type. Present data show that the differences in the responses of the UV-sensitive strains to MNNG from the wild-type strain seem to be most similar to the differences observed in their response to UV with exception of uvs-3 which was stable to mutation induction after UV. The response of these strains to MNNG is different from their response to x-ray irradiation.